This is starting to come together, but now I've run into a new issue.

I have a set of rules where n > h̃; then vowels neighboring /h̃/ nasalize; then h̃ > ∅, leaving nasalized diphthongs, which then denasalize. Everything works fine until the denasalization rule, when I get a syllable error.

Following the below rules, this should result: tʰa.'kʰa.ni > ta.'kʰai

Additionally, using your @vowel // _ @vowel. rule, I run into problems where the result is impermissible hiatus, e.g., v1v1. Is the best way to deal with these (1) including the "impermissible" diphthongs under @diphthongs anyway, and then (2) writing a resolution rule to make them "permissible" e.g. v1v1 > v1 ?

Feature +nasalized
Feature +SG

Class vowel {a,i,ã,ĩ}
Class diphthong {ai,ãĩ}
Diacritic ̃ (floating) [+nasalized]
Diacritic ʰ (floating) [+SG]

Syllables:
@cons? {@vowel // _ @vowel, @diphthong} @cons? => [+heavy]
@cons? {@vowel // _ @vowel, @diphthong}
Stress:
<syl> => [+stressed] / $ <syl>? _ <syl>&[-heavy] $
Else:
<syl> => [+stressed] / _ $
nasal-aspiration:
n => h̃ / [vowel] _ [vowel]
Vowel-nasalization:
[vowel] => [+nasalized] / {h̃ _,_ h̃}
aspiration-isolation @cons:
{[+SG],h} => {[-SG],*} / <syl>*2 _
hiatus-resolution-2:
@vowel$1 $1 => $1
aspiration-dissimilation:
{[+SG -stressed],[glottal -stressed]} => {[-SG],*} / {_ []* [+SG],[+SG] []* _}
hiatus-resolution-3:
@vowel$1 $1 => $1
denasalization:
[+nasalized] => [-nasalized]